A bus network is generally the simplest type of network that consists of a communication line having two end points and zero or more intermediate stubs. Any number of devices may be connected to the communication line. One popular bus network is a Controller Area Network (CAN), developed by Bosch GmbH for the automobile applications in the 1980s. The CAN specification requires that the last two devices at the end points of a bus have termination resistors. These termination resistors are required to ensure proper communication between two or more devices connected to the CAN. In most automobile applications, the end points of the CAN bus are fixed. Additionally, most devices attached to the CAN bus in an automobile environment, such as headlights, dashboard and so forth, are pre-wired.
The CAN protocol has now been adopted to a wide variety of applications. CAN technology is used in some peripheral computer devices, such as printers. The CAN bus may be used to attach optional input and output devices to the printer, such as automated paper trays, bin sorters and many other related input/output devices.
Unlike the devices deployed in most automobile environments, optional input/output devices used in connection with printers are not fixed. That is, such optional devices may be installed and removed from the CAN on a frequent basis. In the case of printers, it is possible for users to install or remove optional paper handling devices. Each time a device is added or removed from the printer, it is possible that the endpoints of the CAN bus may have changed, and hence, termination of the CAN bus must be adjusted to reflect the addition or deletion of a device.
In a non-fixed swappable environment, most optional devices when installed on a CAN must have enough intelligence to determine whether or not they are located on the end-point of a CAN bus. If such an intelligent device, determines that it is located on an end-point, then the device must deploy a termination resistor (i.e., terminate the bus), in accordance with the CAN protocol. Some devices today, use other schemes such as current monitoring on an extra “Power On” line to determine if other devices exist on a CAN bus. So, in cases where optional devices might be connected to the bus, detection of these devices is accomplished by monitoring when a downstream device is turned-on and drawing power.
In cases where optional devices can be plugged-in and out of a printer, on both the input and output sides of the printer, it is common for printers to use two currents sensors to monitor for downstream devices: one sensor for monitoring the input side of the bus and the sensor for monitoring the output side. Using current sensors circuits, however, to detect power consumption on different branches (input/output) of a bus is expensive in terms of design time, cost of hardware elements and customization per product.